Mode filtered cutoff attenuator



Novo 1953 A. E. LAEMMEL 2,659,87O

MODE FILTERED CUTOFF' ATTENUATOR Filed July 10, 1947 INVENTOR.

A T TOPNEY.

Patented Nov. 17, 1953 'UNITED PAT'ENT OFFICE 'MODE FILTERED CUTOFF-ATTENUATOR' 'Arthur L iei, Brooklyn, N. Y., 'assigor t'o rpytechni I'titte os roy, Brooklyn,

N; Y:,- ec 'tration of New York Application July 10, 1947, Serial N. 760, 028

'4 '1ai`s. (Cl. 333-431) This' intention reats t' atterat'or's f the ct=ff type and is' eseecily s'fil for attnuatmg utra high fr'ny ci ents or waves, although it s net liiitd te this' frqeri cy range. Atteniators f the cat-cit type are weil k own. Such ttutors take advantage of the transmission chafctfistic f wave guides for a-tteici tii frequnc'is iwr than a certain e t-oa value One common form' of a cut-cit attenuater i'rivolvs the of a la unchi'ng loop and a 'ree l'op' afffg'd within a wave guide; ad p io i's id fif v'arying the separation between the' twolp. f 'the transverse dimen-'- Sios of the gid ai'f properly chosen,- the power level in db trlsmitted 'to the receiving` loop will vary mv rsy withthe distance of separatiom T w ere it 'is desred s'imly tocntrol the amount use as a primary standard which can be accuf-ty alibrated and will give reproducible resfl s.

The mai'objectof the present invention is to devise an attehuator' of the cnt-oli type' which may be sed as' primary standard, and in which simfl type of attnuato is satisfactcrythe tt'nuatio rate may be predicted accurately The difculty with the simple type of cut-'off ajttet'r described above is that the attema tion characteristic is rendered uncertain and va'bl because a large number of modes of proagatin are present under any given condi t. The attenuationcharacteristic can be 'predictd acc'rately only when a single mode' of p`r` atio is present Accordingl-y, in obtain the bfad general object of my i nveri tion, I pro to devis'e means for filtering out all modes o`f :atio except a single' des'ired mode.

` ei" objects of my invention are to devise* siiit; matching arrangements for the atteiiu' atf; to dvise an attenuator in which the `cut-' off section may become quite shoi' 't without changing the lineari-ty; to devise matching arri'ments for the attenuator for ctieration over a d 'ffequency band and with a minimum ins on less; and to` devise a cutoff attenuati* inwheh the attenuation rate can be .predicted below 'the poi-nt at which it ceass to be a c onstaiit. v

While y inve'ntion may be applied to wave guides of 'tretem shapes, I prefer 'to use a *flat rctariul waveg'iie since this type ofgaide increases 'the width of the frequency band -over which ati act-'dry ration obtained;

My nvefn on wl now b described in cannesti with the amiaying drawing in which Figura ;1 is a-gene'r'al diagrammatic View illustrating the princ'iple of the invention;

igure 2 is aiongitudinal sectional viewof my attenuator applied to a coaxial system;

Figura 2a shows a series of attenuation curvesas applied to Figure 2;

Figure 3 is a longitudinal sectional view of a form -of my attenuator applied to awave guide system; and

Figure i is a sectional View of Figure' 3 taken dominant mode is allowed to prpagate, and the other modes establish fields which decay exponentially,` the rate of decay' being according to the attenuation constant. If the cross sectional dimensions of the wave guide are so selected that the dominant mode does not propagate, then ali excited modes attenuate, and the system becomes a cut-off wave guide; In such a system, the dominant mode possesses the 'lowest rate of anemiaticn or decay In the case of a simple cut-off attenuator re ferred 'to above,` a launching loo'p sets up an expo nentially decaying field, and an adjstable receiving loop arranged within the cut-off guide sectionabsorbs power from the field. in ni opcrtioii to the square of the field intensity at its location.

The' power which is not absorbed by the receiving v loop is -refiected by the launchir'ig loop. Thus,'

may be as high as 30 db. This initial atte nuation' is called the initial insertion loss;

Aspreviously -indicated, 'the main object oi the' s present invention is to eliminate all modes of propagation except a single desired mode. Since the higher modes tend to complicate the attenuation, it is proposed to eliminate the higher modes, and for this purpose it is desirable to use a wave guide having a large ratio between the attenuation constants of the second and first modes. In the circular wave guide, the ratio can be 1.30, referring to the Eo and H modes. In the rectangular wave guide having a height less than twice its width, the ratio is always 2 or greater, referring to Ho and the Hoz. Accordingly, the rectangular wave guide has an advantage over the circular wave guide, and it is preferred that the width of the guide be less than one-half the wave-length of propagation in free ar.

In Figure 1 is diagrammatically shown an arrangement for eliminating the higher modes by means of a rectangular wave-guide section RG interposed between a coaxial input line A and a coaxial output line B. A coupling and matching device M is interposed between input line A and the rectangular wave guide RG, and a similar device N is interposed between the wave guide: and the output line B. The end sections of Wave guide RG are filled with dielectric material as shown in the drawing. If the dielectric material and the size of the wave guide are correctly chosen, all modes except one will be attenuated and cut ofi in the dielectric filled region, and the one mode that is propagated will be attenuated in the unfilled region C-D according to the length of this region. The attenuation and refiection can be calculated down to where the point C coincides with D, since the higher modes from M can never interact with the higher modes from N. Accordingly, all higher modes can be lumped as reactances which are independent of the length C-D. This would not be possible if there were no dielectric sections.

Figure 2 shows one specific Construction applied to a coaxial cable system having an input cable A and an output cable B connected by the rectangular wave guide RG. The cable A is connected to the wave guide through one of the broad sides, and the central conductor of the cable extends into the wave guide to form a launching antenna. The section of the wave guide surrounding the launching antenna is filled with dielectric material E, and high resistance matching film F is arranged transversely across the wave guide within the dielectric filled section.

The output cable B is coupled to the wave guide by means of a. coupling loop G connected between the center conductor of cable B and a movable plug I-I connected to the outer conductor of cable B and slideably mounted within the wave guide. A block of dielectric material I is carried on the front end of the plug H and surrounds the loop G, and a high resistance matching film J is embodied in the dielectric block. The arrangement is such that the plug H with the attached dielectric block I may be moved to provide different separations between the opposed faces of the two dielectric blocks, to thereby vary the length of the attenuating section of the rectangular wave guide. The fact that the air-dielectric interface, the matching film, and the pickup loop all are rigidly connected and move together when the length of the cut-off section is varied means that the impedance looking into the coaxial cable B does not vary (except for a permissable variation at small attenuations due to dominant mode interaction through the cut-ofi section). such an impedance variation if it did exist would cause errors in attenuation measurement which would be quite serious if the device is to be used as a precision ;standard attenuator.

Figure 2a illustrates the manner in which the dierent modes are attenuated within the wave guide of Figure 2. The dotted curve I illustrates the manner in which the dominant mode would Ibe attenuated if the dielectric were not present, while the curve 2 illustrates the transmission obtained for the dominant mode when the dielectric is present. From these curves it will be seen that without the dielectric, the dominant mode is attenuated sharply, while with the di- -electric, the dominant mode is transmitted in the dielectric filled portion without attenuation and then is attenuated in the unfilled section. 'Curves 3 and 4 illustrate the attenuation of higher modes within the dielectric filled section, and it will be seen that these higher modes are substantially cutoff within this section and are not transmitted through the unfilled section.

It is also to be noted that since no sliding contact is required at the ends of the dielectric sections (points C and D in Figure 1) no additional higher modes will be excited at these points. This condition is not obtained in other designs using dielectric filled sections. Of course, a gap 'between the dielectric block and the wave guide wall will also set up higher modes at C and D, but this gap has been exaggerated in Figure 2, and can actually be kept quite small.

By varying the separation between the two dielectric filled sections, the amount of energy transmitted through the wave guide may be controlled in an accurate and predetermined manner.

Figure 3 illustrates an attenuator applied to a wave guide system interposed between a rectangular input guide A' and a rectangular output guide B'. Figure 4 is a sectional view of Figure 3 taken along the line 4 4. In Figure 2 the efiective length of attenuator changes for different adjustments, but in Figure 3 the total length remains fixed. Elements of Figure 3 serving corresponding functions in Figure 2 are indicated by like reference numerals.

A section of the attenuator guide RG adjacent the guide A' is filled with a dielectric block E having a matching film F embodied therein. This film is preferably spaced from the end guide A a distance of one-quarter of the wave-length in the dielectric. The end section of the input guide A' is filled with dielectric as shown at K for the purpose of lowering the characteristic impedance and compensating the eect of the discontinuity at the point where the guide joins the attenuator section. The length of the dielectric filled section K preferably is one-quarter of the wavelength in the dielectric.

The output guide B' is connected by a fixed connection with the attenuator guide section, and a dielectric block I provided with a matching film J is arranged for adjustable movement into the attenuator section. This block of dielectric is carried by a larger dielectric block L mounted for sliding movement in the output guide B' and serving the same purpose as the block K, in the input guide A'. The face of the block L adjoining the block I, and the faces of the block I up to the matching film J, are covered with a low resistance film indicated at O which effectively short-circuits the end section of the output guide B' lying between the block L and the end of the guide. In this section of the guide B', the energy is transmitted through the dielectrc block I to the block L and then to the guide B'. By varying the position of the block L, by any suitable means not shown, the amount of energy transmitted from guide A' to gui-de B' may be controlled in an accurate and predetermined manner.

The fact that the attenuator is approximately matched allows the phase of the reflection to vary, as it will when L is moved, without introduction of variations in the linear attenuation curve. This would not be so if the design were not such as to allow the resistive matching filn J to move with the dielectric blocks I and L.

I claim: i

1. A cut-oli attenuator comprising a rectangular wave guide having a height not greater than one-half the Width, and the width is less than one-halfthe wavelength in free space, a coaxial input cable coupled to the broad face of said guide and having the central conductor thereof extendng into said guide to form a launching antenna for launching wave energy in said Waveguide in a dominant and higher modes, a filling of solid dielectrie material for a section of said guide surrounding said launching antenna, said dielectric filling serving to increase the effective dimensions of the filled section to a point to sustain the dominant mode only while maintaining the waveguide below cut-ofi for said higher modes, and said filled section being of sufficient length to attenuate substantially to Zero all higher modes within said filled section, whereby only the 'dominant mode is transmitted beyond the end of said dielectric-filled section, a pickup element mounted within said wave guide and being adjustable along the length of said waveguide to vary the spacing thereof with respect to said dielectric filled section, and. loss-producing matching means embodied in said dielectric filling at a point between said launching antenna and the end of said dielectric filled section adjacent sai-d pickup element.

2. A cut-off attenuator according to claim 1 and including a block of insulating material surrounding said pickup element and filling a section of said guide between said pickup element and said launching antenna, said dielectric block being movable with said pickup element to vary the length of the unfilled section in said guide.

3. A cut-off attenuator comprising an input waveguide section and an output waveguide section interconnected by an attenuator waveguide section of rectangular cross-section having a height not greater than one-half the width, and a Width not greater than one-half the wavelength of transmission in free air, said output section being of rectangular shape of larger transverse dimensions than said attenuator section, a dielectric filling for a section of said attenuator guide extending from the junction of said input section a distance greater than onequarter wave-length in the dielectric, a lossproducing matching element embodied in said filled section at a point located substantially one-quarter wave-length in the dielectric from the junction of said input section, a block of dielectric material arranged within said output section and being slideable to enter said attenuator section to a variable extent, and a second dielectric block substantially filling a section of said output waveguide and being movable with said first-mentioned dielectric block.

4. A cut-off attenuator comprising a section of uniform Wave guide having a rectangular section having a height not greater than one-half the Width and a width less than one-half the wavelength in free space, a fixed dielectric plug filling one end of said section, a movable dielectric plug filling the other end of said section, said plugs having a dielectric constant greater than that of air an-d serving to increase the effective dimensions of said filled sections to sustain the dominant mode only while maintaining the waveguide below cut-off for all higher modes, a coupling device Secured to the outside end of said movable dielectric plug and being movable with said plug, and loss-producing matching means embodied in each of said dielectric plugs at a point spaced from the end thereof which faces the other plug.

ARTHUR E. LAEMMEL.

References Cite in the file of this patent UNITED STATES PATENTS Number Name Date 2,088,749 King Aug. 3, 1937 2,129,711 Southworth Sept. 13, 1938 2,376,785 Krasik May 22, 1945 2,407,267 Ginzton Sept. 10, 1946 2,423,396 Lindor July 1, 1947 2,427,106 Landon Sept. 9, 1947 2,433,074 Tuller Dec. 23, 1947 2,514,544 e Hansen July 11, 1950 OTHER REFERENCES "Microwave Measurements and Test Equipments" from the "Proceedings of the I. R.. E. and Waves and Electrons" published in October 1946. See page 782. Copy in -183-8.1

"Microwave Transmission Design Data, Pub. No. 23-80. Published by Sperry Corp., Great Neck, L. I., N. Y. Copy in Div. 69.

"Principles of Radar" by M. I. T. Radar School Staff, 2nd Edition. Published in 1946 by Mc- Graw Hill. Copy in Div. 69. 

